The present invention generally relates to an atrial defibrillator for delivering a pulse of defibrillating electrical energy to the atria of a human heart. The present invention is more particularly directed to a fully automatic implantable atrial defibrillator which exhibits reduced power consumption, reliable synchronized delivery of defibrillating electrical energy to the atria, and multiple modes of operation including bradycardia pacing. The present invention is further directed to an improved endocardial lead for delivering the defibrillating electrical energy to the atria while minimizing the electrical energy applied to the ventricles. The present invention is still further directed to lead systems for use in an atrial defibrillator and method for monitoring activity of the heart and delivering cardioverting or defibrillating electrical energy to the heart.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness or even loss of consciousness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistent to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators have become a commercial reality.
Implantable atrial defibrillators proposed in the past have exhibited a number of disadvantages which probably has been the cause of these defibrillators from becoming a commercial reality. Two such defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these defibrillators require the patient to recognize the symptoms of atrial fibrillation with one defibrillator requiring a visit to a physician to activate the defibrillator and the other defibrillator requiring the patient to activate the defibrillator from external to the patient's skin with a magnet.
Synchronizing the delivery of the defibrillating or cardioverting energy with an electrical activation (R wave) of the heart is important to prevent ventricular fibrillation. Ventricular fibrillation is a fatal arrhythmia which can be caused by electrical energy being delivered to the heart at the wrong time in the cardiac cycle, such as during the T wave of the cycle. As a result, it is most desirable to sense electrical activations of the heart to generate synchronization pulses (or signals) in a manner which avoids detecting noise as an electrical activation. Unfortunately, implantable atrial defibrillators proposed to date have not provided either such noise immunity or any other means for assuring reliable synchronization.
Another measure for reducing the risk of inducing ventricular fibrillation during the delivery of defibrillating electrical energy to the atria of the heart is to reduce the amount of the electrical energy which is passed through the ventricles. In other words, it is advantageous to confine the electrical energy to the atria as much as possible.
Implantable defibrillators, in general, must be powered by a portable, depletable power sources, such as a battery. However, an automatic implantable atrial defibrillator which continuously monitors atrial activity of the heart and which continuously monitors for atrial fibrillation will consume so much power that frequent battery replacement, requiring explanting the defibrillator, would be necessary.
The atrial defibrillator of the present invention provides solutions to all of the above noted deficiencies in atrial defibrillators proposed to date and other features which obviate potential problems in implantable atrial defibrillators. In general, the atrial defibrillator of the present invention is fully automatic and provides reliable synchronization to electrical activations, both through noise immune electrical activation sensing and through a test mode which permits a physician to confirm reliable electrical activation sensing. The atrial defibrillator of the present invention also provides for conserving battery power by activating the atrial fibrillation detector only when the ventricular rate indicates a probability of atrial fibrillation. In addition, the atrial defibrillator of the present invention provides a new and improved endocardial lead and a method for using the same which assures that the delivered electrical energy is confined to the atria and little of the electrical energy is passed through the ventricles.
In addition to the foregoing, the lead systems and method disclosed herein reduce battery power consumption and hence lengthen the useful life of an implanted atrial defibrillator employing such lead systems. The lead systems disclosed herein are configured for placing the cardioverting or defibrillating electrodes in the heart at locations which minimize the energy which must be delivered to the atria for cardioverting or defibrillating the same. Furthermore, the cardioverting or defibrillating energy levels disclosed herein are intended to provide a fifty percent probability of successful defibrillation or cardioversion. This is based upon the recognition that atrial fibrillation is not generally life threatening and that if a second delivery of cardioverting or defibrillating electrical energy is required for successful cardioversion or defibrillation, the life of the patient will not be threatened. The end result is less battery power consumption, extended life of the atrial defibrillator, and of greatest importance, less frequent surgical replacement of the atrial defibrillator to provide the patient with greater comfort and less risk commonly attendant to all surgeries.